A mass spectrometric study of the effect of varnish removal from a 19th century solvent-sensitive wax oil painting

Preliminary cleaning test of a nonrelined painting by the Dutch impressionist painter W. Maris (1844-1910) revealed that the paint softened considerably upon exposure to solvents. Multicomponent sensitive direct mass spectrometric (MS) techniques were used to identify organic components in microgram sized samples taken from selected paint layers. The unknown yellow tinted varnish was identified as a slightly aged methylcyclohexanone resin of the AW2 type. Residues from a previously removed oxidised Dammar varnish were also identified. The impasto paints were oil with beeswax which makes these paints sensitive to all solvents. The effect of a fast solvent based varnish removal technique (maximum contact time 2 s) was investigated by mass spectrometric analysis of the paint layers. The isooctane/ethanol (1:1) removed the varnish and fatty acids from the top layers of the paint but did not affect the wax esters and alkanes from the beeswax. The organic composition of the deeper paint layers was not changed. MS studies of the ground layer suggests that during a previous cleaning, fatty acids, diacids, diacylglycerols and a small amount of the beeswax have migrated to the highly absorbent chalk; ground which has a yellowed somewhat transparent fatty appearance.